The invention relates generally to fluid control and, more particularly, to automated control over fluid delivery pressure.
Infusion pumps are used to more precisely control the infusion of medical fluids into the vascular system of a patient. Syringe pumps are one type of an infusion pump in which the syringe plunger is moved into the syringe barrel at a controlled rate to administer the syringe contents to the patient. Such pumps are typically used to administer the infusion fluid generally as opposed to locally. That is, the medical fluid is placed into the bloodstream of the patient for general distribution throughout the body rather than applying the medical fluid only to a localized delivery site in the body. Consequently, such pumps are typically controlled to deliver the medical fluid in accordance with a desired flow rate rather than at a precisely controlled pressure.
While many such pumps include a pressure sensor, the sensed pressure is typically used only to trigger pressure alarms that will shut off the flow from the syringe pump if an occlusion or other undesirable condition is detected. The pressure provided by the pump is often only controlled to reside in a particular pressure range, which may be relatively wide, and to remain below a predetermined upper limit; the range and upper limit being selected in dependence upon the particular application.
However, in the case of local drug delivery systems for injecting medical fluids into the walls of blood vessels, into body organs or other internal delivery sites, pressure control is a primary fluid delivery parameter. In some cases, it is desirable to treat a disease by locally applying a medical drug in a high concentration. The concentration may be so high that the drug could cause damage to other parts of the body or even be life threatening if allowed to freely enter the blood stream of the patient, yet the drug may have the desired effect if confined to a local application. In such local applications, it has been found that pressure control is important. Excessive pressure may cause a dissection of the vessel wall and too little pressure will not force the medical fluid into the vessel wall but may allow it to be swept away by the bloodstream before it can be delivered to the desired location.
Studies have indicated that the application of particular medical fluids to the vessel walls following the performance of an angioplasty procedure on those walls has the potential of impeding restenosis at that site. For example, see Wolinsky et al., Local Introduction Of Drugs Into The Arterial Wall: A Percutaneous Catheter Technique, Journal of Interventional Cardiology, Vol. 2, No. 4, 1989, pgs 219-228. These studies indicate that the application pressure of the medical fluid to the vessel walls contributes greatly to the success or failure of the treatment. Thus, it would be of value to precisely control the pressure of the medical fluid applied in such a procedure.
The inflation balloons used in such catheter based, local introduction techniques typically include a certain number and size of apertures through which the medical fluid will be delivered to the vessel walls. The balloon is positioned at the desired location in the vessel and pressure is applied to inflate the balloon. However, the apertures tend to impede a rapid inflation because they allow a part of the inflation pressure to escape. Additionally, the apertures may allow some of the inflation fluid to escape into the bloodstream during inflation. In the case where the fluid to be applied to the vessel walls has some higher level of toxicity to the patient if the fluid should enter the bloodstream, a more rapid balloon inflation is desired. One means of achieving a more rapid inflation is to apply a higher pressure during initial inflation to force the balloon into its operative configuration as soon as possible and then lower the pressure to maintain the target pressure. An automated system would be desirable to achieve this control over inflation.
Additionally, while the balloon is inflated, and even when only partially inflated, the blood flow is interrupted or at least impeded. To avoid injury to the patient, a time limit on the inflation is imposed. Therefore, full inflation of the balloon should occur as rapidly as possible so that administration of the medical fluid can be started as soon as possible so that all of the medical fluid can be applied to the delivery site before expiration of the time limit.
More precise pressure control is also desired where the medical fluid applied to the vessel walls has some higher level of toxicity to the patient if the fluid should enter the bloodstream. In this case, it is necessary to maintain the balloon at a predetermined pressure throughout the application of the drug to lessen its chances of entering the bloodstream. The balloon must be kept at a high enough pressure so that the apertures formed in the balloon are in extremely close contact with the vessel walls. That is, the balloon cannot be permitted to deflate to an extent where the apertures are exposed to the bloodstream so that the medical fluid would be taken from the delivery site by the blood flow. By this means, drags which may be somewhat toxic if applied to the patient through the bloodstream may be locally applied to the walls of a blood vessel to perform a post-angioplasty or other function while not adversely affecting the patient.
In one type of prior inflation/deflation system, a syringe is attached to the proximal end of a catheter containing the balloon and a pressure gauge is located adjacent the syringe to measure the pressure of the medical fluid in the catheter. The plunger of the syringe is manually moved into the syringe barrel to expel the fluid contents from the syringe through the catheter and into the vessel walls through the balloon apertures. The pressure indicated on the pressure gauge is monitored by the operator during the movement of the syringe plunger and the operator varies the movement of the syringe plunger in an attempt to maintain the desired pressure. It has been found that manual methods such as this typically do not adapt quickly enough to compensate for pressure variances as the fluid is being delivered into the vessel walls. Pressure variances may be caused by various factors including site geometry, blood pressure, the number of apertures in the balloon, catheter geometry, and the viscosity of the medical fluid being applied, for example.
A drop in the pressure indicated on the pressure gauge may stimulate the operator to accelerate the movement of the plunger into the syringe barrel which may result in a pressure spike. High pressures have been found to result in necrosis of the inner media of the vessel (Wolinsky et al., id. ) and if too high, dissection of the vessel. Low pressures have the effect of transmitting the drug into the bloodstream, as discussed above. Thus, it is desirable to provide a system which permits a more rapid response time with more precise control over the pressure.
A further consideration in such systems is deflation and removal of the delivery device. In the case where the delivery device comprises an inflatable balloon having apertures for applying a medical fluid, deflation should occur so that the drug remaining in the balloon is captured by the catheter, rather than being released into the blood stream. Applying a pressure below the pressure of the delivery site, which in some cases may require a negative pressure, should occur relatively rapidly so that the medical fluid is not released into the bloodstream. The pressure in the system may change as the balloon is collapsed, thus monitoring the pressure and correcting it as it varies during removal of the balloon is desirable. Applying a negative pressure which is too large will unnecessarily draw blood or other body fluids into the catheter while too little negative pressure may allow the medical fluid to enter into the bloodstream.
In an angioplasty system in which a catheter having an inflatable balloon is positioned at a delivery site for applying pressure to the vessel walls by inflating the balloon, more precise pressure control may also provide a benefit. Although this is a closed system in that the inflation fluid is not continuously leaving the system as in the drug delivery system, pressure must be monitored for proper performance of the procedure. Excessive pressure may cause damage to the vessel walls while insufficient pressure will not expand the vessel walls enough to successfully accomplish the angioplasty.
It has also been found desirable in many drug delivery systems to more precisely control the volume of the medical fluid delivered along with more precise pressure control. In some applications, the volume of the drug to be applied to the delivery site, such as blood vessel walls, is very small (less than five milliliters). Present fluid injection systems have not permitted the precise control over pressure in the delivery of such a small amount of fluid.
Hence, it has been recognized by those skilled in the art that a more accurate fluid pressure control system is desirable. It has also been recognized that a system which provides automated control over pressure during the delivery of fluid is also desirable. It has been further recognized that a system which accurately controls pressure during the local application of medical fluid while avoiding infusion of the medical fluid into other areas is desirable. The present invention fulfills these needs and others.